Wear Merino in Hot Weather

Wear Merino in Hot Weather

by Kyle Barraclough

Can You Wear Merino Wool in the Summer?

Yes, you can wear Merino wool comfortably in the summer. Surprised? Don't be. The Bedouins have been doing it for centuries in the Sahara.

Merino Wool Fiber: Cool When Hot and Hot When It’s Not

Usually, when people think of wool, the first thought that comes to mind is a warm wool garment on cold, rainy or snowy days. But, that reputation is only half the equation. Wool’s breathability and its amazing natural performance characteristics render it useful, comfortable and functional in almost any kind of weather – including warm climates or hot summers.

While that may sound counterintuitive, consider the ceiling fan as an example. In the summer, by turning the fan blades to blow upward, warm air near the ceiling is pushed down the walls mixing with cooler air near the floor helping keep the room cool. And this is the common image that most people have of the ceiling fan – that of a cooling mechanism. But reverse the blades in the winter and you get the opposite effect. Warm air from the ceiling is pushed to the center of the room and cooler air is drawn upward, balancing the temperature in the room and making the room warmer and more comfortable.

Like the two-way functionality of the ceiling fan, wool is a dynamic fiber that naturally adjusts to conditions around it. And all it takes to dispel the feeling that wearing it in summer to stay cool is counterintuitive is a quick look at the science and the benefits that make it an ideal garment for both winter and summer.

Thermoregulation - Merino Wool’s Balancing Act

As a body builds heat during physical activity, some of that heat is radiated outward from the body. In addition to radiation, the body will also begin to produce sweat to induce evaporative cooling.

When being physically active in the heat, some garments may actually hold moisture against the body and reduce or prevent the evaporative cooling effect. This is even true of cotton garments, long considered the go-to for “activewear”. But, Merino wool is different; sweat can move through the fibers due to the porosity of the fiber structure.

Wool moves the perspiration vapor through the spaces between the fibers themselves. At the microscopic level, wool fibers have a natural “crimp”. This crimp is a wavy characteristic that remains to a degree even after the fiber is spun into yarn. The result is spaces where moisture can move.

The moisture moves along the longitudinal axis of the fibers by way of capillary action. The unique chemistry of wool fibers interacts with hydrogen in the water. This effect creates a differential in surface tension that allows water to move along the fibers, including upward and outward. This process is commonly known as “wicking”, where moisture is wicked away from the skin to allow the wearer to remain cooler.

Wool naturally attracts moisture through a process known as absorption. But unlike other fibers, wool can also release moisture through a process called desorption, meaning that once it has absorbed the sweat from the skin and the moisture has transited through the fibers, it can release the moisture outward thus shedding the moisture from the body. Studies by researchers such as Leeder in 1984 and Collie and Johnson in 1998 have shown that wool can absorb up to 35% of its weight in a humid environment before it begins to feel wet.

Because of this unique combination characteristics, wool garments used in the summer can experience sudden changes in heat from the wearer and absorb water quickly from the inside while releasing it more slowly on the outside. And again, because wool is a dynamic fiber, it works in reverse as well, absorbing quick changes in humidity so the wearer experiences none of the clamminess often associated with humidity changes.

These actions in combination can keep the surface of the skin cooler longer during active periods. Like the ceiling fan, wool’s natural versatility, unique chemical structure and moisture transportation can actually cool you as well as warm you, depending on the condition and climate making Merino fabric ideal for any season.

Merino Compared to Other Summer Wear and Activewear Fabrics

When comparing Merino with other fabrics, it's all about the fiber. Merino is a natural fiber with a unique chemical composition and inherent performance characteristics. For example, the aforementioned natural crimp that assists in moisture removal and migration also adds bulk. Fibers such as polypropylene and acrylic must have crimp mechanically added to give it the same loftiness and this loftiness may not hold over time as will Merino.

Man-made fibers can have wicking capability if the structure is created during manufacturing. Usually, the fibers are extruded in hair like strands and either cut or stretched. They may be oval, star-shaped, or triangular to help produce the ability to wick moisture away – something Merino does naturally.

Merino even outperforms cotton by comparison as well. While cotton is highly absorptive, it does not have the ability for desorption. This means that once a wearer begins to sweat, the water absorbed by cotton holds the water to the body. Since cotton cannot regulate temperature, there can be discomfort. For example, when the temperature drops after sweat has soaked the cotton shirt, the retained water can become cold and cause a drop in the core body temperature. This is important to travelers that experience rapid temperature and altitude changes. The decrease in body temperature is not only uncomfortable, but it can decrease energy and compromise the immune system.

Clothing Systems, Comfort & Homeostasis

Evolution is about survival. And a big part of that is the ability to keep a balanced and stable temperature relative to surroundings, previously identified as “thermoregulation.” There are automatic physiological responses to balance body temperature which protect humans to a point. But, beyond that point we use clothing layers to complement the natural processes. But, not all clothing fabric has the same capability to protect the wearer. The wrong fabrics can actually cause harm in both hot and cold weather.

Evolution has also provided critical involuntary physical responses to ambient temperature. If we experience extreme physical activity or a situation of extreme heat, the human body produces sweat. To put this in perspective, at rest humans may produce a half a liter of moisture per day. But in extreme heat, that rate increases dramatically and the body can produce almost a liter of sweat per hour.

The body has also developed responses to extreme cold, with rapid muscle spasms that define shivering to generate heat when the body’s core temperature is lower. We may also experience goose bumps where tiny muscles under the skin push body hair upright to help hold warm air closer to the skin.

These responses are all part of our evolved mechanisms to allow thermoregulation. This is important because they affect our attempts to maintain a constant internal balance called homeostasis. This balance is achieved when the voluntary and involuntary responses work together.

Each environmental change triggers a receptor that notifies us of that fact. This may be a blast of cold as we walk into a cool airplane off the hot tarmac. Or, it could be a wave of heat as we sit at a cookout on a remote beach. The receptor tells us we need to do something to alter the condition.

The control center processes the info from the receptor and triggers a response through an effector. This could be a voluntary response such as removing oneself from the situation or putting on a layer or a blanket or removing a layer. It could also be an involuntary response such as goosebumps or chills.

Thermoregulation is critical because it helps the body maintain a comfortable environment and balance the internal heat. In fact, it is so critical that if our environment becomes too extreme and remains as such, we experience stress or anxiety that can make us sick. Or, if the temperature is too extreme to overcome, we could experience heat stroke or hypothermia.

Types of Heat Transfer

Just as staying comfortable during extremes of heat and cold is a challenge, so is staying comfortable during long journeys. Since the body is always striving to maintain a stable temperature and inner environment, it comes down to heat management. Heat is transferred by one of three methods.

  • Conduction - The body touches something and transfers heat to it or has heat transferred to the body.
  • Convection – Heat is transferred through vapor such as sweat or breath.
  • Radiation – Heat escapes the body to seek cooler regions and will rise as it does so.

Layering Wool Clothing for Temperature Control

All these methods can be affected or controlled by wearing thicker or thinner layers. In extreme cold environments, high loft fabric with a lot of space between layers will help maintain heat. While in warm environments, thin Merino wool garments can help move heat effectively through the fabric and away from the body.

When layering, the ideal is to have a structured layering scheme that suits the environment. This can be done through using a three-layered system consisting of the following three layer types: base, mid, outer (see below for examples of these layers).

Thermal comfort exists when your brain tells you it's happy with the temperature and environment around it. When heat transfer occurs through fabric, thermal comfort can change for good or ill. Heat can also be transferred through fabric by all the methods of heat transfer - conduction, convection or radiation.
All fabrics have specific thermal properties and benefits depending on end use. These include:

  • Thermal Conductivity – How well the fabric breathes.
  • Heat Resistance – The inability of fabric to breathe because it is acting as a barrier.
  • Thermal Resistance – Fabric is constructed in such a way that it will resist heat transfer for a period of time.

When choosing layers, the functionality of the layers and the surrounding environment combine to define the comfort level.

Base Layer:  Wick moisture away from skin.

Business travel base layers

  • Garments: Short sleeve undershirts
  • Fabric Construction: Knitted wool, cotton, man-made & synthetics
  • Micron range: 15.5-17.5
  • Weight range: 120-160 gsm

Outdoor/Holiday travel base layers

  • Garments: Long sleeve “base layer” and sometimes common sportswear can serve as a substitute.
  • Fabric Construction: Knitted wool, cotton, man-made & synthetics
  • Micron range: 15.5-17.5
  • Weight range: 120-160 gsm

Mid Layer: Thermoregulation by the creation of space between base layer and outer environment.

Business travel mid layers

  • Garments: Dress shirts or business polos
  • Fabric Construction: Knitted or woven of wool, cotton, man-made & synthetics
  • Micron range: 15.5-18.5
  • Weight range: 130-220 gsm

Outdoor/Holiday travel mid layers

  • Garments: Flannels & overshirts. Mostly the button-up variety.
  • Fabric Construction: Mostly woven wool, cotton, man-made & synthetics
  • Micron range: 17.5-18.5
  • Weight range: 145-250 gsm

Outer (Shell) Layer: A barrier to rain, snow, wind and other conditions that can affect heat retention.

Business travel outer layers

  • Garments: Wool suits, sportcoats, overcoats
  • Fabric Construction: Woven of wool, cotton, man-made & synthetics
  • Micron range: 15.5-21
  • Weight range: 180-300 gsm (Some heavy overlayers can be 545 gsm (16 oz) to 950 gsm (28 oz)

Outdoor/Holiday travel outer layers

  • Garments: Often called “shells” or “outer shell.”
  • Fabric Construction: Mostly woven of high-tech fabric
  • Micron range: Not usually measured in micron
  • Weight range: MFG stated applicable temperature range is more important/applicable

Merino Wool for the Savvy Traveler

Fortunately, modern society means that extremes of heat and cold are experienced much less often than in past centuries. But that doesn’t mean that environments along travel routes are always comfortable. The constant shifting of humidity and temperature, combined with the effects of alternating between natural environmental changes and manmade ones such as hotels, aircrafts and transportation, can still impact people psychologically and physically and will still trigger responses to seek comfort. As a result, the search is always on for comfortable travel clothing.

Since Merino wool apparel both cools and warms depending on the environment, it can not only be worn year-round, it is an ideal choice for travel because of the rapid climate changes during a typical travel day. Travelers can rest easy knowing that the same garment that helps keep them cool when standing in a stuffy airport concourse will also keep them warm on the plane and dry when they leave the hotel heading to the beach. It also means packing less since Merino garments can be used cross functionally. That means less packing and luggage, or alternately, more room for bringing other refinements that are often left behind because of clothing needs.

Merino Wool’s Comfort & Protection Extends Beyond Temperature Control

In addition to the superior and dynamic management of heat and moisture discussed in this article, Merino also provides a higher degree of safety. According to studies by researchers Hilfiker in 1996 and Reinert in 1997, it offers much better UV protection across the entire UV spectrum compared to other fabrics.

Merino wool is also naturally flame resistant. In addition to its relative moisture content, Merino is also made up of approximately 14% nitrogen which raises its ignition temperature and gives it a tendency to self-extinguish.

Many of the benefits of Merino are health related. With an active moisture and heat management fabric, the wearer is less stressed and able to maintain a balanced and stable temperature. This not only reduces stress; it also helps reduce the chance of exhaustion which can lead to illness.

Many who wear Merino for activewear will also notice less irritation on the skin. The protection afforded by the moisture and heat regulation means that skin is less chafed and less exposed to prolonged moisture that can lead to bacterial buildup. The same is true of footwear where Merino socks have been shown to help reduce the chance of blisters.

Merino wool is versatile and stylish. It is also breathable, manages heat and moisture transfer and provides a high degree of comfort in both warm and cool environments. This is due to Merino’s unique chemical structure that allows it to operate much like that ceiling fan, dynamically managing moisture levels, protecting the wearer from multiple environmental concerns such as UV rays and offering comfortable functionality that can go with you on any journey.

References:

  1. https://www.dhhs.nh.gov/tips/documents/stayingwarm.pdf
  2. http://scienceline.ucsb.edu/getkey.php?key=4600
  3. https://pdfs.semanticscholar.org/1050/b9f7aa6cc19af3f4d6865bdf1f76c08970c3.pdf
  4. https://www.technicaltextile.net/articles/clothing-and-thermal-comfort-6626
  5. http://windowoutdoors.com/WindowOutdoors/Staying%20Warm%20-%20Interesting%20Observations%20on%20Heat%20Loss.htm
  6. https://xaktly.com/HeatTransfer.html



Kyle Barraclough
Kyle Barraclough

Author

Founder, Libertad Apparel



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